1. Field of the Invention
The present invention relates to a distillation apparatus that carries out a distillation operation widely applied to many industrial processes, and more particularly to a heat integrated distillation apparatus.
2. Description of the Related Art
Distillation separation is a unit operation widely applied to industrial processes in general, but consumes a large amount of energy. In the industrial field, therefore, studies have been conducted on an energy saving distillation systems. Such studies have brought about development of a heat integrated distillation column (hereinafter, HIDiC) as a distillation apparatus that save much energy.
As shown in FIG. 1, a basic system of the HIDiC has a structure in which a rectifying section (high-pressure unit) and a stripping section (low-pressure unit) are provided such that they are separate from each other. Operation pressure of the rectifying section is set higher than that of the stripping section so that the operation temperature of the rectifying section can be higher than that of the stripping section. This enables a reduction in the amount of heat that is supplied to a reboiler because heat transfer occurs from the rectifying section to the stripping section when there is a heat-exchange surface therebetween. Heat of the rectifying section moves to the stripping section, and hence the amount of heat that is supplied at a reboiler can be reduced. As a result, high energy saving distillation apparatus can be achieved.
In order to put the concept of HIDiC to practical use, a number of distillation apparatuses having double-pipe structures, that is, double-pipe structures constituted of inner pipes forming rectifying sections and outer pipes forming stripping sections have been proposed (For example, refer to JP2004-16928A). These configurations are described as being capable of reducing the amounts of heat that are supplied to the reboilers and the amounts of heat that are removed at the condensers, since heat transfer occurs from the rectifying sections (inner pipes) to the stripping sections (outer pipes).
However, the heat integrated distillation apparatus having the rectifying section and the stripping section formed into the double-pipe structures as discussed in JP2004-16928A had the following problems 1) to 6).
1) The product cannot be obtained with side-cut stream. The side-cutting means that a product is withdrawn as an intermediate distillate product, during a distillation process until an end distillate is acquired from top of column.
In the distillation apparatus described in JP2004-16928A, the tube units of the double-pipe structures are arranged to come into contact with each other. Moreover, the outer pipes and the inner pipes are equipped with the structured packing. As a result, no pipe arrangement can be formed to withdraw any intermediate distillate product from the inner pipe of each tube unit. Consequently, the structure disables side-cutting.
2) The feed stage where feed stream is provided cannot be optimized. This is because in the rectifying section and, the stripping section formed into the double-pipe structures, packing heights thereof are equal, disabling free setting of the number of stages of the rectifying section and the stripping section.
3) The feed stage cannot be changed so as to meet the feed stream composition. This is because of the structure in which free setting of the feeding stage position is disabled as described in 2).
4) Multi-feed stream (reception of a plurality of feed streams) cannot be dealt with. This is because of the structure in which no feed stream can be supplied in the midway of the double-pipes as described in 1).
5) Maintenance of the apparatus is difficult. The tube units that use the structured packing are densely arranged to be adjacent to each other as described in 1). This disables complete access to the desired tube unit, and maintenance thereof cannot be carried out.
6) The heat exchanged rate between the rectifying section and the stripping section that uses double-pipes and in which there is no a degree of freedom in design for designing the heat transfer area, depends only on the temperature profile of the distillation column. Hence, in apparatus design, a degree of freedom in design of heat exchanged rate is small.
Q, the heat exchanged rate between the rectifying section and the stripping section, is represented by Q=U×A×ΔT, where U is an overall heat-transfer coefficient, A is a heat transfer area, and ΔT is a temperature difference between the rectifying section and the stripping section. In the HIDiC using the double-pipe structure, an inner pipe wall surface becomes a heat transfer area. This heat transfer area has a fixed value determined by a structure of the double-pipes. The overall heat-transfer coefficient also has a fixed value determined by the heat transfer structure and fluid physical properties involved in heat exchange. Thus, as can be understood from the heat exchanged rate formula, a heat exchanged rate on design specification can be changed based only on the temperature difference between the rectifying section and the stripping section, which is changed by the operating pressure of the rectifying section and the stripping section.
As the heat integrated distillation apparatus that can solve the problem as described above, the present applicant has proposed the apparatus of JP4803470B.
FIG. 2 shows an example of the distillation apparatus disclosed in JP4803470B. The distillation apparatus includes rectifying column 1, stripping column 2 located higher than rectifying column 1, first pipe 23 for connecting column top 2c of the stripping column with column bottom 1a of the rectifying column, and compressor 4 that compresses vapor from column top 2c of the stripping column to feed the compressed vapor to column bottom 1a of the rectifying column. The distillation apparatus further includes liquid sump unit 2e located at a predetermined stage of stripping column 2 and configured to hold liquid that has flowed downward, heat exchanger 8 located in liquid sump unit 2e, partition plate 16 that is set in a predetermined position of rectifying column 1 and configured to apart upper stages and lower stages completely, second pipe 29 for introducing vapor below partition plate 16 to heat exchanger 8, and third pipe 30 for introducing fluids introduced through second pipe 29 to heat exchanger 8 and then discharged out of heat exchanger 8 to an upper side of partition plate 16.
With the above described configuration, the vapor is withdrawn from rectifying column 1 through second pipe 29. The vapor is introduced to heat exchanger 8 in stripping column 2. Then, heat can be transferred from rectifying column 1 to stripping column 2. High-pressure vapor in rectifying column 1 ascends through second pipe 29 to heat exchanger 8 in stripping column 2. A fluid partially or totally condensed from the vapor in heat exchanger 8 is accordingly pushed out from stripping column 2 to third pipe 30 outside the column. Thus, this configuration also necessitates no pressure-feeding means such as a pump in supplying liquid from stripping column 2 to rectifying column 1 located at a lower side in a vertical direction.
Further, with the above described apparatus configuration, which transfers heat from rectifying column 1 to stripping column 2 by using second pipe 29, third pipe 30 and heat exchanger 8, as compared with a distillation apparatus including no such heat transfer configuration, the heat exchanged rate removed from condenser 7 attached to the column top of rectifying column 1 can be reduced more, and the heat exchanged rate that is supplied to reboiler 3 attached to the column bottom of stripping column 2 can be reduced more. As a result, a distillation apparatus that is very high in energy efficiency can be provided.
Rectifying column 1 and stripping column 2 can be configured by using trayed sections or packed bed sections similar to those of a general distillation apparatus. Hence, the apparatus can deal with side cutting or multi-feed stream without the need for any improvement, and it is possible to easily perform maintenance of the apparatus. For the same reason, the number of stages of the rectifying column or the stripping column can be freely set, and a feed stage can be optimized.
A heat transfer area can be freely set, and hence the heat exchanged rate can be determined without any dependence on the temperature difference between the columns.
As described above, according to the device example described in JP4803470B (FIG. 2), energy efficiency is high, side-cutting and setting of a feed stage position can be easily dealt with, and maintenance of the apparatus is easy. The apparatus of the present invention has a structure in which a degree of freedom in design is high, and hence can be easily accepted by the user side.
Concerning the distillation apparatus shown in FIG. 2, the present inventors aim at further enhancement in energy efficiency, and consider that the distillation apparatus still has a room to be improved.
In other words, in the distillation apparatus shown in FIG. 2, the following method is adopted. Partition plate 16 that completely partitions the inside of the column to an upper side and a lower side is installed in an arbitrary stage of rectifying column 1, all of vapor ascending from below partition plate 16 is withdrawn from the column through pipe 29, and is supplied to tube-bundle-type heat exchanger 8 installed at an arbitrary stage of stripping column 2, where heat exchange is performed. Thereafter, a fluid partially or totally condensed in heat exchanger 8 flows through pipe 30 outside the column to the upper side of partition plate 16 in rectifying column 1 by gravity, and the condensed liquid flows through another pipe 31 to be movable to below partition plate 16. Such circulation of the fluids is performed.
Such a method intend to withdraw all of the vapor in rectifying column 1 to the outside of the column, and hence adopts a complicated structure in which partition plate 16 is installed in rectifying column 1, and the condensed liquid fed onto partition plate 16 from stripping column 2 is further transferred to a lower side space of partition plate 16 through pipe 31 and control valve 17 outside the column. Thus, there is a room to be improved from the viewpoint of the structure and manufacturing cost.
Further, drive force for the fluids passing through the tube of heat exchanger 8 is obtained by giving pressure loss at the upper and lower sides of partition plate 16, and hence pressure of column bottom 1a needs to be made larger than pressure of column top 1c of rectifying section 1 correspondingly to the pressure loss at the upper and lower sides of partition plate 16. Thus, there arises a need for setting pressure to be higher at an outlet side of compressor 4 (namely, increase a compression ratio) correspondingly to increase in the pressure at column bottom 1a side. Therefore, there is also a room to be improved from the viewpoint of energy saving performance.